# LeetCode 895. Maximum Frequency Stack

## Description

https://leetcode.com/problems/maximum-frequency-stack/

Implement `FreqStack`, a class which simulates the operation of a stack-like data structure.

`FreqStack` has two functions:

• `push(int x)`, which pushes an integer `x` onto the stack.
• `pop()`, which removes and returns the most frequent element in the stack.
• If there is a tie for most frequent element, the element closest to the top of the stack is removed and returned.

Example 1:

```Input:
["FreqStack","push","push","push","push","push","push","pop","pop","pop","pop"],
[[],,,,,,,[],[],[],[]]
Output: [null,null,null,null,null,null,null,5,7,5,4]
Explanation:
After making six .push operations, the stack is [5,7,5,7,4,5] from bottom to top.  Then:

pop() -> returns 5, as 5 is the most frequent.
The stack becomes [5,7,5,7,4].

pop() -> returns 7, as 5 and 7 is the most frequent, but 7 is closest to the top.
The stack becomes [5,7,5,4].

pop() -> returns 5.
The stack becomes [5,7,4].

pop() -> returns 4.
The stack becomes [5,7].
```

Note:

• Calls to `FreqStack.push(int x)` will be such that `0 <= x <= 10^9`.
• It is guaranteed that `FreqStack.pop()` won’t be called if the stack has zero elements.
• The total number of `FreqStack.push` calls will not exceed `10000` in a single test case.
• The total number of `FreqStack.pop` calls will not exceed `10000` in a single test case.
• The total number of `FreqStack.push` and `FreqStack.pop` calls will not exceed `150000` across all test cases.

## Explanation

keep track of the frequency, max frequency, and what are the elements of each frequency

## Python Solution

``````class FreqStack:

def __init__(self):
self.frequency = collections.Counter()
self.frequency_groups = collections.defaultdict(list)
self.max_frequency = 0

def push(self, x: int) -> None:
frequency = self.frequency[x] + 1
self.frequency[x] = frequency

if frequency > self.max_frequency:
self.max_frequency = frequency

self.frequency_groups[frequency].append(x)

def pop(self) -> int:

value = self.frequency_groups[self.max_frequency].pop()
self.frequency[value] -= 1

if not self.frequency_groups[self.max_frequency]:
self.max_frequency -= 1

return value

# Your FreqStack object will be instantiated and called as such:
# obj = FreqStack()
# obj.push(x)
# param_2 = obj.pop()``````
• Time Complexity: ~1
• Space Complexity: ~N